The invention relates to a method for driving light-emitting diodes of an illumination device, in particular an illumination device for film, video and photographic recordings with a pulse-width-modulation of the light-emitting diode current and to a device for driving light-emitting diodes of an illumination device, in particular an illumination device for film, video and photographic recordings with an electronic switch for the pulse-width-modulation of the light-emitting diode current flowing through the light-emitting diodes.
Illumination spotlights comprising light-emitting diodes (LEDs) or light-emitting diodes as light sources in spotlights in film, video and photographic recordings are known, the significant advantages of which consist in the long lifetime, significantly less evolution of heat by comparison with other light sources such as incandescent lamps or HMI lamps, for example, a lower weight and a compact design in conjunction with a high luminous intensity that is output.
A further significant advantage of light-emitting diodes as light sources in illumination spotlights consist in the fact that the color rendering and/or the color temperature can be set by using light-emitting diodes of different colors. This advantage is of importance particularly in the case of film and photographic recordings with light-sensitive film material, since typical film materials for film recordings such as “cinema color negative film” are optimized for daylight with a color temperature of 5600 K or for incandescent lamp light with a color temperature of 3200 K and achieve excellent color rendering properties with these light sources for the illumination of a set.
DE 102 33 050 A1 discloses an LED-based light source for generating white light which makes use of the principle of three-color mixing. In order to generate the white light, the three primary colors red-green-blue (RGB) are mixed, at least one LED which emits blue light and which is referred to as transmission LED and emits directly used light primarily in the wavelength range of 470 to 490 nm and also another LED, which operates with conversion and is accordingly referred to as conversion LED and which emits light primarily in the wavelength range of at most 465 nm, being combined in a housing.
One disadvantage in the capability of setting the color rendering and/or color temperature by using different-colored light-emitting diodes in illumination spotlights for film, video and photographic recordings consist, however, in the fact that the emission wavelengths of the light-emitting diodes and hence the color rendering and color temperature depend greatly on the temperature, which essentially depends on the ambient temperature and the current flowing through the light-emitting diodes. One possibility for keeping the color rendering and color temperature constant consists in regulating a color correction in a current-dependent manner. Such regulation can only be realized with high outlay, since it would have to be associated with an additional, temperature-dependent compensation.
In order to make the emission wavelength of light-emitting diodes independent of the current flowing through the light-emitting diodes, the brightness of the light emitted by the light-emitting diodes is controlled by means of a pulse-width-modulation and the current flowing through the light-emitting diodes is kept constant.
DE 102 27 487 A1 discloses an illumination device comprising a plurality of series-connected light-emitting diodes, for the driving of which a microprocessor as pulse-width-modulator is provided, which enables pulsed operation of the series-connected light-emitting diodes, such that the light-emitting diodes can be operated within a very short time with a multiple of the current otherwise permitted and both the illuminance and the lifetime of the light-emitting diodes can thereby be increased. The output of the microprocessor is connected to a power driver, which amplifies the output signals of the microprocessor and outputs the required electrical power to the series-connected light-emitting diodes. On the input side, the microprocessor is connected to an analog/digital converter, which is connected via a connecting line to an electrode terminal of the light-emitting diodes and detects a voltage that is dropped across the light-emitting diodes and is proportional to the ambient brightness, which voltage is converted into a digital signal and evaluated by the microprocessor. In this case, the frequency of the pulse-width-modulated current output by the microprocessor is in the region of 25 Hz in order that the pulsation of the light-emitting diodes is no longer perceptible to the human eye as such as is identified as continuous brightness.
A pulse-width modulation with such a low frequency, which is also increased up to 100 Hz or a few kHz in other applications, is inadequate particularly for film and video recordings, however, since motion-picture or video cameras also with relatively short exposure times for each frame to be exposed of, for example, 1/48 seconds at a film transport rate of 24 frames per second, which are reduced down to a few 1/10000 seconds at higher film transport rates and small sector angles of a rotating, sector-variable mirror diaphragm of a motion-picture camera. As a result, in the case of a low-frequency pulse-width modulation for driving light-emitting diodes of an illumination device, severe exposure fluctuations occur during the exposure of the individual film or video frames, which are unacceptable for good-quality film and video recordings but also for photographic recordings with very short exposure times.
EP 1 638 205 A1 discloses an LED driver circuit with constant-current regulation and pulse-width modulation which contains a constant-voltage source with output voltage regulator, which sets the output voltage depending on an external signal, a switching circuit connected to the cathode side of the LEDs, the switch-on and switch-off times of said switching circuit being controlled in pulse-width-modulated fashion, a current detection unit, which detects the current flowing in the circuit, and a sample-and-hold circuit, which holds the input value of the current detection unit over a constant period and outputs it as input signal to the output voltage regulator.
However, the use of a sample-and-hold circuit or a comparable circuit arrangement such as a peak-hold circuit, at high frequencies of the pulse-width modulation, owing to the small pulse widths of the pulse-width-modulated light-emitting diode current and owing to the ever present inductances of the circuit and the not exactly rectangular light-emitting diode current resulting therefrom, is not sufficiently representative of the pulse-width-modulation current flowing through the series-connected light-emitting diodes since a current value of the current detection unit is detected at a specific instant or with regard to its maximum. In order to comply with the desired color or color temperature of the illumination device even in the event of changes in the ambient temperature or temperature of the illumination device and thus to avoid color fluctuations in film recordings, a maximally exact current regulation and thus exact detection of the pulse-width-modulated current is necessary.